The principal object of the present invention is to provide a self-averaging transverse probe which can be easily installed in duct stacks for measuring gas flow in both clean and contaminated mediums, with high resolution, high accuracy and requiring little straight run or flow preconditioning.
There have been attempts to measure bulk air flow by using multiple mass air flow sensors directly within the air stream. The drawbacks of prior systems include sensors that are easily contaminated by the measured medium and must be cleaned on a regular basis by either removing the flow device, or physically entering the duct and cleaning the individual sensors. Critical placement of the mass sensors by an appropriate equal average weighted sensing pattern is frequently difficult, since the thermal sensors cannot always be placed close together due to size constraints. Compromises must then be made that affect the number of sensors which may be used and the ability to position each of them in a proper location. Likewise, since thermal mass measurement can only detect a change of air flow, calibration for the actual amount of air passing through the conduit must be obtained by measuring the air flow by some other method. The most common method in large conduits is a pivot tube. A further disadvantage is that another air flow device is required by the prior art systems as a calibration device. These problems are disadvantages of using mass air flow measurement technology However, the benefits of greater air flow resolution, accuracy and the elimination obtained by pre-flow conditioning (straight run) still makes this technology of continued interest, if some of these disadvantages can be eliminated.
U.S. Pat. No. 4,487,062, issued Dec. 11, 1984 relates to a mass flow meter having a primary passage for fluid whose flow rate is to be measured and a sensor tube which is parallel to the primary passage. Resistance wire coils surrounding the sensor tube and connected to a bridge circuit provide a resistance differential between the wire coils that is proportional to the fluid flow rate. Replaceable plugs permit cleaning of the sensor tube. However, the patent has no teaching of a valve for calibration purposes; pickup and discharge tubes which are used to provide a proportional flow to the sensor and are transverse to the primary flow passage or any means to clean portions of the meter which may become clogged without disassembly or entrance into the conduit.
U.S. Pat. No. 4,559,835, issued Dec. 24, 1985 discloses a transverse probe of a design particularly adapted to measure the flow of air or other gas in a conduit. The probe is of a specific design which permits both the pressure (inlet) and vacuum (outlet) elements to be incorporated in a unitary elongated construction. The patent contains no disclosure of any sensor description or construction.
U.S. Pat. No. 4,648,270, issued Mar. 10, 1987 discloses a mass flow meter having a bypass channel and a detector channel. Thermistors are used in the detector channel to sense the flow being measured The signal is produced by a differential amplifier connected to the two thermistors, and a linearization circuit is also disclosed. There is no disclosure of a mass flow meter having transverse probes, or a valve permitting calibration and/or alternative pressure differential measurement of flow rate in a conduit. Furthermore, there is no teaching of a means to clear pickup ports or the like, and it is particularly to be noted that metal screens or porous discs such as those taught to provide a flow restriction in the main conduit could only be cleaned by entering the main conduit or removing the entire metering assembly, thus shutting down the main conduit.